Radio receivers receive and process radio frequency transmitted energy signals to yield, generally, audible information. Typically, the originally received RF signal will be converted to an intermediate frequency (IF) signal prior to reducing the desired signal information yet further to audio levels. Various signal processing functions may occur at the IF level, including signal attenuation as may be necessary to prevent the signal from exceeding a threshold level.
With reference to FIG. 1A, one prior art attenuation circuit for providing such attenuation can be seen to include two coupling capacitors (A and B), a current source (C), and a diode (D). Typically, diodes present an impedance of 26 ohms at one milliamp. This impedance, however, will diminish as current flow through the diode increases For example, with 10 milliamps flowing through such a diode, the impedance presented by the diode will be approximately 2.6 ohms. Therefore, by regulating the flow of current through the diode (D) by appropriate control of the current source (C), the impedance of the diode (D) can be manipulated to govern attenuation of a signal as received at the input.
The above prior art solution presents various problems. Distortion can result with high input signal levels. Further, impedance of the diode (D) cannot be reduced much below, for example, 2.6 ohms in a specific device. Also, because a DC component that moves with the carrier can lead to unsatisfactory signal processing, the two capacitors (A and B) must be provided to guard against this eventuality. In an integrated circuit context, the use of such capacitors poses significant problems with respect to design, manufacture and cost. Further, the response of this circuit tends to be temperature dependent, and this raises important design and performance issues
FIG. 1B depicts another prior art attempt at providing a satisfactory attenuation circuit. This circuit again includes the two capacitors (A and B), the current source (C), and the diode (D) of the circuit described above. In addition, this circuit includes a third capacitor (E), a second diode (F), and two low impedance resistors (G and H). This configuration has the advantage of lessening distortion, due to the balanced diode configuration. This circuit also has disadvantages, however. In particular, this circuit requires the low impedance resistors (G and H) on either side of the two diodes (F and D) and also requires the additional capacitor (E). Further, the DC component can still move with current and temperature variations.
There therefore exists a need for an attenuation circuit that can provide satisfactory attenuation with less distortion than currently offered by prior art techniques. There further exists a need for an attenuation circuit that avoids the problems associated with DC movement and the necessity of providing both coupling capacitors.